The present invention relates generally to special effects generators for television signals, and more particularly to an electrical mechanism for generating a shadow for a color television chroma key effect.
In the development of television signals for transmission to devices such as receivers, it is often desired to combine signals from two or more independent sources. At the present time, a wide variety of special effects generators are being used by, say, commercial telecasters to select the available signal inputs in a variety of ways. For example, manual fading and lap dissolve between signal sources wherein an operator having control of the gain of the channels feeding into a mixer unit may switch rapidly from one channel to another or gradually fade one out and the other in, either allowing the signals to blend or be faded at any desired rate. Another method of changing from one channel to another is the well known wipe which allows the simultaneous presentation of two pictures, an example of which is the case in which a variable width blanking gate is used to wipe out all or part of the signal in the channel and replaced by a signal from another channel, the opposite part of which is blanked out. Other special effects generators can be used to combine signals from different channels to produce artistic effects, time share signals, etc. Such special effects generators are the so called keyed insertions whereby an arbitrary portion of one channel signal is blanked out and may or may not be replaced by signals from another channel depending on the effect desired. As an example of prior art methods and processes to produce these artistic effects including shadows, those interested are referred to U.S. Pat. No. 3,788,542 by L. C. Hanseman. This patent, which is an improvement of the basic work in the art, elaborates on a chroma key process called Technimatte for combining RGB components of the signals to be mixed to provide a chroma key produced by a proportional use of the amount of color information in one of the signals to be mixed. Other prior art processes such as Imagematt and Chromatech, developed by Image Transform Inc., and Technicolor, Inc. respectively, provide similar processes and methods. These processes can only produce shadows using color difference information. Therefore, none of these conventional special effects generators produces or is capable of producing a true shadow that is developed from luminance information. Shadows and variations in light intensity on, say, a real background that gives the real scene depth are lost in a standard chroma key effect whereas the present invention shadow key process and method retrieves these shadows and light intensity variations from the original scene and impresses them on the new background providing a three-dimensional effect that greatly improves the appearance of a linear chroma key.